Nickel is widely used in steel alloys and as metal plate. Nickel salts are well-recognized as allergens and carcinogens. Their potential effects upon human male fertility have not been thoroughly explored, but rodent studies show Ni2+ induces reversible testicular atrophy and spermatogenic arrest. In preliminary studies, even though nanomolar Ni2+ concentrations have no effect upon human sperm motility, supplementing in vitro incubation media with nanomolar Ni2+ markedly decreased human sperms' response to acrosome reaction inducers, a bioassay that is an excellent predictor of human sperm fertilizing potential. The current goal is to determine if there is a correlate in human populations to this in vitro result, and, if so, to begin to dissect the molecules directly targeted by Ni2+. Levels of Ni2+ in human blood and seminal plasma will be correlated with in vitro fertilization, with artificial insemination rates, and with biomarkers of sperm function, using specimens collected in the course of prior studies of metal ion toxicants. Levels of Ni2+ will also be contrasted in stockpiled human testis biopsies of infertile and fertile men. An effect size (variance of the mean divided by the within-group variance) of 0.1 will be detectable using specimens currently on hand. Confounding effects of cofactors (including levels of hormones and other metal toxicants, conventional semen analysis parameters, and lifestyle factors) will also be assessed to dissect effects due to Ni2+. Consequences of perturbed calcium levels (possible increases in apoptotic cells in testes and decreased polymerized action in germinal epithelia) will be determined to assess the likelihood that Ni2+ toxicant action parallels mechanisms demonstrated for Cd2+ and Pb2+. As the somatic cell literature and preliminary experiments with an ion channel blocker suggest that T-type voltage-gated Ca2+ ion channels offer both means of Ni2+ entry into germ cells and an early target for toxicant action, expression of T-type Ca2+ ion channel isoforms in human testicular mRNA will correlated with fertility status to determine if isoform expression is a biomarker for increased sensitivity to Ni2+ as a reproductive toxicant, in a similar manner as expression of L-type Ca2+ ion channel isoforms is a biomarker for testicular toxicity of Cd2+.